1. Field of Invention
This invention relates generally to electronic assemblies and more specifically to the attachment of electronic components to substrates.
2. Background of the Related Art
Electronic assemblies are traditionally manufactured by attaching components to substrates, such as printed circuit boards. The substrates provide mechanical support for the components and have signal paths that electrically interconnect the components. In printed circuit boards and other types of substrates, signal paths between components are provided by conducting strips, called “traces.” Often, traces are internal to the printed circuit board so holes, called “vias,” are needed to extend from a surface of the printed circuit board to the traces. The vias are plated with conductive material to create an electrical connection between a component on the surface of the printed circuit board and a trace within the board.
The mechanism attaching components to the substrate should have desirable electrical and mechanical attributes. The attachment should electrically connect components to the vias in a way that provides little distortion of electrical signals passing between the component and traces of the substrate. Further, the attachment between the component and the substrate should be mechanically robust so that the electrical connection is not disrupted by forces on the interface between the component and the substrate as the electronic assembly is used. Many types of attachments have been used.
Early electronic assemblies were manufactured using a through-hole solder attachment technique. With this form of attachment, leads from components on the front side of a printed circuit board are inserted through the vias. Solder is applied to the back of the printed circuit board, often by dipping the leads in a solder bath. Molten solder tends to adhere to the metal of the lead and the plating of the via. Attractive forces between the molten solder and the lead draw the solder along the lead in a process sometimes called “wicking.” When the solder cools and hardens, it makes an electrical connection between the lead and the plating of the via, and it also secures the lead in the via.
Press-fit connections have also been used. A press-fit connection also uses a via for attachment but relies on force generated by a contact tail to couple the contact tail to the via. A press-fit lead is stamped with a contact tail that has a compliant section. The compliant section is compressed as the lead is inserted into the via. Once inside the via, the complaint section generates a spring force against the walls of the via. The force creates both an electrical connection and a mechanical connection between the contact tail and the walls of the via.
More recently, the use of surface mount techniques has become prevalent. With surface mount techniques, vias are also used to make connections to traces or other conductors such as ground planes or power planes within the printed circuit board. The vias serve only as conducting paths between pads on the surface of the printed circuit board and traces internal to the printed circuit board. Because the vias do not receive leads or contact tails from components to be attached, the vias can often be made smaller in diameter than those used for through-hole or press-fit attachment. Smaller diameters allow the vias to be placed closer together or be positioned to allow more traces to be routed between vias in the area of the substrate where components are mounted. Either effect can lead to a smaller electronic assembly. Smaller diameter vias can also improve electrical performance.
Electronic components are attached by soldering leads from the components to the pads on the surface of the substrate. Such leads are often stamped from flat pieces of metal and then bent or “formed” into shapes. Commonly used shapes include “gull wing” leads and “J-leads.” Though, in some instances, the leads may be simply posts that are not formed. Regardless of the shape, the leads are typically soldered to the pads using a reflow solder process.
In a reflow process, solder paste is positioned on the pad. Solder paste is viscous enough to hold a lead loosely in place when a component is placed on the board. Once components are placed on the board, the board is placed in an oven that heats the solder paste.
A fluxing agent and solder particles within the solder paste are transformed during heating. As the solder paste is heated, the fluxing agent becomes activated. At the beginning of the reflow process, the flux attacks oxide and other contaminants on the surfaces of the pad and the lead being interconnected. The flux also “wets” the surfaces to promote solder adhesion. As the flux is heated more, it turns into a gas that should escape from the solder paste. Simultaneously, the solder particles within the paste melt. The molten solder adheres to both the lead and the pad. When the molten solder cools, it solidifies to electrically and mechanically join the lead to the pad.
Surface mount techniques have also been developed using solder balls. In many cases, electronic components attached with solder balls do not have leads. Instead, both the component and the substrate have pads that align. Solder balls are placed between the pads and reflowed to secure the pads on the component to the pads on the substrate. Solder paste or flux may be used to hold the solder balls in place. As with other surface mounting techniques, the solder balls are reflowed and molten solder adheres to the pad on the substrate and the pad on the component. When the solder cools, it forms an electrical and mechanical connection between the pads.
Many variations of solder ball mounting are known. In some variations, the solder balls have solid cores, such as copper spheres. The spheres shape the solder joint and establish a spacing between the component and the substrate when soldered.
Surface mount techniques are often used when very high density interconnections are desired. Because there is no need for access to the pads to make a solder joint, arrays of pads can be formed on a substrate, and a component may be placed over the array of pads. Many electronic components are manufactured with an array of solder balls to align with such an array of pads. These components are often said to include “Ball Grid Array” (BGA) packaging.
The above-described attachment techniques have generally employed solder, which contains lead (Pb) (Because the word “lead” may refer to a material or a portion of an electronic component, where necessary to distinguish, the symbol Pb following the word “lead” indicates that the word refers to a material.). Because lead (Pb) is regarded as a hazardous material, electronic assemblies formed using solder attachment may require special processing when their useful life is exceeded and the electronic assemblies are discarded. The use of conductive adhesives in place of lead (Pb)-based solder has been proposed as a way to avoid disposal difficulties associated with electronic assemblies containing lead (Pb)-based solder. It would be desirable to have an improved attachment mechanism that does not involve lead (Pb)-based solder.